1. Field of the Invention
The present invention relates to a variable attenuation circuit that is used for a television tuner or the like.
2. Description of the Related Art
A variable attenuation circuit according to the related art has a configuration in which a first attenuator unit 41a, an RF amplifying circuit 42, and a second attenuator unit 41b are connected in cascade, as shown in FIG. 2. Both attenuator units 41a and 41b have the same circuit configuration and thus only the first attenuator unit 41a will be described. A PIN diode D1a is provided between an input terminal INa and an output terminal OUTa of the first attenuator unit 41a. An anode of the PIN diode D1a is connected to the input terminal INa via a connection capacitor C1a and a cathode of the PIN diode D1a is connected to the output terminal OUTa via a capacitor C2a. 
The anode of the PIN diode D1a is applied with a control voltage v2 via a driving resistor R1a, a driving transistor Tr1a, and a high-pass cut-off coil L1a. The cathode of the PIN diode D1a is grounded via a resistor R2a. 
Further, the anode of the PIN diode D1a is grounded via a direct current (DC) blocking capacitor C3a, a PIN diode D2a, and a capacitor C4a. An anode of the PIN diode D2a, that is, the connection point between the capacitor C3a and the PIN diode D2a, is applied with a predetermined voltage v4a. The voltage v4a is generated, for example, by dividing a predetermined power supply voltage B by the resistors R3a and R4a, and is set to be lower than a voltage v3a on the cathode of the PIN diode D1a, when the control voltage v2 is the maximum. A cathode of the PIN diode D2a is connected to the cathode of the PIN diode D1a via a PIN diode D3a. 
Then, as the control voltage v2 is changed, the collector current of the transistor Tr1a is changed, such that the current flowing in the PIN diodes D1a to D3a is controlled (for example, see Japanese Unexamined Patent Application Publication No. 2000-013162 (FIG. 1)).
In the attenuator unit 41a, the current flowing In the PIN diodes D1a to D3a is controlled by the driving transistor Tr1a, such that the attenuation amount is changed. In this case, when the attenuation amount is insufficient, the attenuator unit 41b having the same configuration as the attenuator unit 41a needs be further provided. Accordingly, the circuit size becomes large and thus the tuner or the like cannot be reduced in size.